Over recent years, from interest in energy conservation, the reduction of power consumption in a variety of devices and efficiency improvement of a power supply system are being promoted. Also for a power supply apparatus such as an AC/DC conversion apparatus and the like mounted in a variety of devices, efficiency improvement of power conversion is needed. In a common power supply apparatus, efficiency characteristics of power conversion tend to vary with an output power (a magnitude of a load). In a low load region where the output power of the power supply apparatus is small, a consumption of power necessary to drive the power supply apparatus itself appears as lowering factors (a loss) of power conversion efficiency. On the other hand, in a high load region where the output power of the power supply apparatus is large, output current increases and therefore, a loss due to an impedance of a power conversion circuit in the power supply apparatus, and iron loss, copper loss, and the like appear as lowering factors.
In response to such variations of power consumption, improvement of power conversion efficiency of the entire power supply system is needed. To improve power conversion efficiency, there is known a technique for operating a main power supply unit using commercial power or the like as a main power supply source and an auxiliary power supply unit (a sub power supply unit) using a secondary battery as a power supply source in combination therewith. For example, related arts prior to the present application are disclosed in the following patent literature.
Patent literature 1 (WO 2002/061917) discloses a technique including a main power supply unit for outputting DC power using a commercial AC voltage and an auxiliary power supply unit for supplying power during power failure using a secondary battery. In the technique disclosed in the patent literature 1, output power from the auxiliary power supply unit is also supplied to a load during a time other than power failure according to power consumption consumed by a load.
Patent literature 2 (Japanese Laid-open Patent Publication No. 2005-291607) discloses a technique regarding a heat pump apparatus for supplying power to a load by combining a power generation unit using an engine and an external power supply using a secondary battery. Patent literature 2 discloses a technique for optimizing power generation efficiency in the power generation unit by controlling power supply from the external power supply and a charge power to the external power supply in the heat pump apparatus.
As a technique for improving power conversion efficiency in response to a variation of power consumption, a technique that changes the number of power supply apparatuses to be operated, and a technique that switches different types of power conversion apparatuses to be operated are known.
For example, Patent literature 3 (Japanese Translation of PCT International Application Publication No. 2013-504986) discloses a technique, for a power supply system including a plurality of power supply apparatuses being connected in parallel to a load, that the operation number of the power supply apparatuses is switched according to power consumption consumed by the load to prevent a decrease in a power conversion efficiency of the entire power supply system.
The power conversion efficiency is expressed by a ratio of an output power to an input power. In connection therewith, the applicant of the present application has proposed a method for calculating an input power by using a measurement of an output power, that is easily measurable, in Patent literature 4 (Japanese Laid-open Patent Publication No. 2011-022022).